Abstract

The separation of soma from germ may have originated as a result of a specialization in source and sink, with somatic cells acting as sources, gathering nutrients from the external environment and germ cells as sinks, utilizing nutrients to grow and reproduce. This hypothesis can be tested in an organism, such as Volvox, where single germ cells can be cultured in isolation from the soma, thus serving both as source and sink, and their growth compared with that of germ cells with an intact soma where source and sink are separated into different cells. Results from such an experiment show that germ cells grown with an intact soma had greater rates of increase than those grown with disrupted soma or that were completely isolated, but the difference became greater as nutrient concentration increased, as predicted by the source-and-sink hypothesis. The advantage, however, was not sufficient to compensate fully for the initial investment in soma, especially at low nutrients, perhaps due to the energetic cost of swimming. In nature, species with segregated soma are found in nutrient-rich lakes and ponds. In experimental farm ponds, the biomass of such species increases with eutrophication more than the biomass of related species without division of labour, suggesting an advantage consistent with the source-and-sink.